The 525 has a lot of ports to choose from. Physically you have 2 and you can add up to at least 3 more fiber connections via an add-on card (probably 6 b/c there are dual cards out there).
This design has a single point of failure. If the firewall goes down you are hosed on both networks.
If you don't have much traffic the 501's are pretty cheap these days, relatively speaking. If this is a big production network then you may consider getting 2 of the 525's and running one in passive/failover mode. You do not need a full license for that one, it reads its config off of the other one and takes over when the first one fails.
